1. Field of the Invention
The present invention relates to traveling wave tubes and, more particularly, concerns the slow-wave structure of such tubes and a method of manufacture.
2. Description of Related Art
In traveling-wave tubes, a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner that amplifies energy of the electromagnetic wave. To achieve desired interaction between the electron stream and the electromagnetic wave, the latter is propagated along a slow-wave structure, such as an electrically-conductive helix that is wound about the path of the electron stream. The slow-wave structure provides a path of propagation for the electromagnetic wave that is considerably longer than the straight axial length of the structure, so that the traveling electromagnetic wave may be made to propagate axially at nearly the same velocity as the electron stream.
Slow-wave structures of the helix type have been supported within a tubular housing by means of a plurality of longitudinally disposed dielectric rods that are circumferentially spaced about the slow-wave helix structure. Some supporting assemblies have employed a coaxial helix of dielectric material wound in the same sense as and aligned with the slow-wave structure helix, positioned between the slow-wave structure helix and the housing.
A helical supporting arrangement for a slow-wave structure is disclosed in U.S. Pat. No. 3,670,196 to Burton H. Smith. The Smith patent employs a mandrel having a helical groove in which is formed both the conductive helix and the overlying dielectric helix.
U.S. Pat. No. 4,115,721 to Walter Friz shows an arrangement similar to that of the Smith patent, but instead of mechanically winding a helical dielectric member in the mandrel groove, the dielectric material is plasma-sprayed into the groove, after which the materials are precision ground to a desired radial dimension.
U.S. Pat. No. 4,005,321 to Arthur E. Manoly, assigned to the assignee of the present invention, discloses an arrangement in which a plurality of axially-extending boron nitride support rods of rectangular cross section are disposed about the surface of the slow-wave helix structure, between the envelope and the helix.
U.S. Pat. No. 4,229,676 to Arthur E. Manoly, assigned to the assignee of the present invention, describes a slow-wave structure with a helical dielectric support.
U.S. Pat. No. 2,851,630 to Charles K. Birdsall describes a traveling-wave tube with a helical slow-wave structure having a gradually decreasing pitch that causes axial velocity of the traveling wave to decrease in a manner corresponding to decrease in axial velocity of the electron stream.
U.S. Pat. No. 3,972,005 to John E. Nevins, Jr., et al describes a traveling-wave tube that is provided with a conductive loading arrangement that increases bandwidth.
The helix support structures of the prior art exhibit a number of problems. Axially-extending support rods are slender and brittle, and therefore difficult to handle. They tend to be easily broken into smaller sized pieces. Particularly because they extend across the inter-turn spacing of the conductive helix, they adversely affect dielectric loading. Moreover, structures which employ three or four axially-extending rods provide relatively low capacity thermal paths between the outer tubular envelope and the conductive helix.
Methods of forming helical dielectric supports involve a number of complex processing steps and are difficult to accomplish with precision, particularly for the higher frequency devices wherein circuit components are exceedingly small.
Where a type of comb support structure has been used, employing a plurality of circumferentially-spaced, longitudinally extending rails to which are attached dielectric blocks spaced so as to contact the successive turns of the conductive helix, difficulties are encountered with obtaining proper registration of the blocks with the turns of the conductive helix. These difficulties are increased where velocity taper is employed so that helix pitch changes near the output end of the structure. Moreover, thermal capacity, namely the ability of the support structure to conduct heat from the electrically-conductive helix to the tubular support envelope, is limited.
Accordingly, it is an object of the present invention to provide a slow-wave structure and dielectric support therefor which avoids or minimizes above-mentioned problems.